The invention relates to an electric apparatus comprising a housing formed by at least a first part containing a switched-mode power supply, a second part containing a processing circuit and a shield for isolating said first and second parts, said shield having at least an opening at which a filter device is installed.
The invention also relates to a cable distribution network comprising at least an electric apparatus of this type.
The invention notably applies to amplifiers used in the cable television signal distribution systems.
The TONNA company, for example, is marketing such an apparatus referred to as 945331. In this apparatus the filter device comprises a circular capacitor passed through by a wire and mounted on a metallic support which is itself attached to the shield.
Such circular capacitors are marketed, for example, by the Californian company INSTEC under references P/N610-103A1 and P/N640-103B.
They have the drawback of being expensive and cumbersome when their attenuation capacity is large. For this reason, the capacitance of the circular capacitors used is generally limited to 10 nF.
Such an attenuation is not always sufficient. It is particularly not sufficient to filter voltages delivered on the output of the switched-mode power supply. A preliminary filter circuit is thus provided at the switched-mode power supply to limit the random noise in the supply voltages delivered at the output of the switched-mode power supply. The circular capacitor placed at the shield only has for its function to attenuate the random noise radiated at the power supply, which are the result of the wire connecting the switched-mode power supply to the processing circuit.
The invention notably has for its object to propose a filter device which has a large attenuation capacity, of which the cost and cumbersomeness is limited.
An electric apparatus according to the invention and as described in the opening paragraph is characterized in that said filter device is formed by a printed circuit which comprises:
connection means for receiving a signal coming from said first part and for delivering a filtered signal to the second part,
at least one ground plane,
at least a capacitor of the surface-mounted type, arranged between said connection means and the ground plane,
means for connecting ground to the shield.
The invention thus permits to obtain high-value capacitances at reduced cost and cumbersomeness. Since it is possible to utilize considerable attenuation capacitances, it is no longer necessary to provide a preliminary filtering at the switched-mode power supply.
By way of example, a circular capacitor as used in the prior art, which operates at a voltage of 50 V and which has a value of 10 nF costs about 2 euros. A circular capacitor of the same type having a value of 1 xcexcF costs about 15 euros. In a printed circuit of 25 mm by 15 mm it is possible to realize various filters for filtering various signals by using surface-mounted capacitors which have a value of the order of 1 xcexcF or more at a cost of about 1.5 euros.
One of the advantages of the invention is thus to permit the number of filtered signals to be multiplied without this significantly increasing the cost of the device.
Furthermore, the circular capacitors currently available on the market have a maximum value of 1 xcexcF. The invention permits to obtain values which are much higher (of the order of 100 xcexcF).
In a particularly efficient embodiment said printed circuit is a double-faced circuit with via holes in which the capacitor(s) is (are) mounted on one of the faces and the ground plane(s) is (are) mounted on the other face. The printed circuit is, for example, flush mounted on the shield by one or various leaf springs so that the ground(s) is (are) in contact with the shield.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.